Irene roars into life; may become the season's first hurricane

Tropical Storm Irene roared into life this evening, transitioning from a tropical wave to a 50 mph tropical storm in just a few short hours. An Air Force hurricane hunter aircraft in the storm was finishing up its mission when it suddenly came across a region with intense thunderstorms and surface winds of 50 mph. The aircraft found that a center of circulation had barely closed off on the southwest edge of this region, though the plane found almost no winds from the west around the circulation center. The 6:10pm EDT center fix found a central pressure of 1007mb, which is quite high for the observed 50 mph winds. Dry air to the north and west is slowing development, as well as moderate wind shear of 10 - 20 knots, as analyzed by the University of Wisconsin CIMSS group. Infrared satellite loops and radar out of Martinique show the storm is poorly organized, with no evidence of spiral bands. The center of Irene is expected to cross over the Caribbean island of Dominica early Sunday morning, but the heaviest thunderstorms lie to the north of the center, and will affect Guadeloupe, Antigua, and St. Kitts and Nevis.

Figure 1. Evening satellite image of Irene.

The computer models have shifted southwards since yesterday, and now take Irene south of Puerto Rico on Monday, and along the south shore of the Dominican Republic on Tuesday. On Wednesday, Irene should pass near or over southern Haiti, Eastern Cuba, and Jamaica. On Wednesday and Thursday, the models agree that a trough of low pressure will dip down over the Eastern U.S., which is likely to turn Irene to the north. The exact timing and strength of this trough varies considerably from model to model, and will be critical in determining where and when Irene will turn to the north. We can expect that Irene will impact Central Cuba, the Bahamas, and the Florida Keys on Thursday, but it is uncertain whether Irene's turn to the north will take the storm into the Gulf of Mexico or not. Irene most reminds me of Tropical Storm Fay of 2008. Fay formed just off the coast of Puerto Rico, and was never quite able to get organized enough to become a hurricane, due to passage over Hispaniola and Cuba. Fay topped out as a strong tropical storm with 70 mph winds, and did over $500 million in damage in the U.S., mostly due to flooding rains in Florida that accumulated to over 25 inches in a few areas. Fay also dumped heavy rains on Hispaniola, triggering flooding that claimed eight lives.

Irene will be battling dry air and moderate wind shear of 10 - 20 knots through Sunday, and it will take the storm 1 - 3 days to build up an eyewall and intensify into a hurricane. Irene is more of a threat than Tropical Storm Emily of early August was, since Irene has closed off a center farther east than Emily did and has more time to organize before encountering Hispaniola. I don't think passage over Hispaniola will destroy Irene, since it is a fairly large storm, and is likely to be a hurricane by then. However, if Irene follows the NHC forecast, it will have an extended encounter with Hispaniola and Cuba on Tuesday through Wednesday that will probably weaken the storm below hurricane force. Keep in mind that the average error for an official 5-day forecast from NHC for a developed storm is 200 - 250 miles. Irene could easily miss Florida and move up the East Coast and hit North or South Carolina, or pass through the Florida Keys and into Gulf of Mexico, ending up who knows where. Given the uncertainties, this weekend would be a good time to go over your hurricane preparedness if you live anywhere in the Caribbean, Bahamas, or U.S. coast, since Irene could well be paying you a visit as a tropical storm or hurricane sometime in the next week.

Harvey hits BelizeTropical Storm Harvey made landfall at 2pm EDT on Saturday near Dangriga Town, Belize, as a tropical storm with 60 mph winds. Harvey continues to dump very heavy rains on northern Guatemala, Belize, and portions of Mexico's Yucatan Peninsula as the storm tracks westwards at 12 mph. Dissipation is expected Sunday as the storm pushes inland. Harvey was a small storm, and the strongest winds were confined to a short stretch of coast near where the center came ashore. Winds at Belize City, Belize on Saturday topped out at 15 mph.

Figure 3. Radar image of Harvey taken at 11:30am EDT on Saturday, August 20, 2011, a few hours before landfall in Belize. A small closed eye is visible just south of the offshore islands of Belize. Image credit: Belize National Meteorological Service.

An exceptionally active early part of hurricane seasonIt's been a strangely hyperactive season for weak storms in the Atlantic so far this year. Tropical Storm Irene is the 9th named storm this year, and its formation date of August 20 ties 2011 with 1936 as the 2nd earliest date for formation of the season's 9th storm. Only 2005 had an earlier date. The first eight storms this year have stayed below hurricane strength, making 2011 the first hurricane season since record keeping began in 1851 to have more than six consecutive tropical storms that did not reach hurricane strength. As I discussed in Friday's post, a major reason for this is the lack of vertical instability over the tropical Atlantic so far this year. We've had a large amount of dry, sinking air over the tropical Atlantic, and the usual amount of dry, dusty air from the Sahara, both helping to keep the atmosphere stable and stop this year's storms from intensifying into hurricanes. Hurricane activity typically ramps up big-time by August 20, with more than 80% of all the hurricanes and 65% of all the tropical storms occurring after that date. At our current pace, 2011 will become the second busiest Atlantic hurricane season on record, with 24 - 27 named storms. There are only 21 names in the list of names for a hurricane season, so we may have to break out the Greek alphabet again in late October this year, as occurred in 2005. Ironically, this was the last time the current set of names was used in the Atlantic, so 16 of this year's 21 names are repeats of 2005. I'm not too happy about seeing another hurricane season challenge the Hurricane Season of 2005 in any way, and let's hope we don't retire another five names this year, like occurred in 2005! With vertical instability much lower this year than in 2005, and that year having already seen one storm (Dennis) retired by this point in the season, I doubt that will happen, though.

Figure 2. The annual cycle of average hurricane frequency in the Atlantic. Historically, about 35% of all the tropical storms and 15% of all the hurricanes will have occurred by August 20.

Invest 98L near the Cape Verde IslandsA tropical wave near Cape Verde Islands off the coast of Africa, Invest 98L, is spreading heavy rains and strong gusty winds to those islands today. So far this morning, top sustained winds measured in the Cape Verde Islands were 23 mph at Mindelo. 98L has a long stretch of ocean to cross before it could affect any land areas. Approximately 70 - 80% of all tropical cyclones that pass this close to the Cape Verde Islands end up curving out to sea and not affecting any other land areas, according to Dr. Bob Hart's excellent historical probability of landfall charts. The latest set of long-range model runs go along with this idea, and I'd be surprised if 98L threatens any land areas.

Quoting Culterbayor:Here in Dorado Puerto Rico on vacation. Go fiqure storm warnings are going to follow me all the way home to Miami. Puerto Rico has a "not a big deal perspective on storms". Resort is good to go with back up generators and lots of free coffee...

Its beautiful there, we used to have a villa in dorado. enjoy your vacation and stay safe

FWIW, the TWC personally thinks it will go right in between Miami and Naples. They never actually said this, I was just looking at the wind directions in south florida's local forecasts. It sorta seems like it exits into the eastern GOM around Tampa but it's hard to be exact going off of these windshifts.

I dont see this coming to florida.... I think its more of a Puerto Rico-Hispaniola and Bahamas anda threat to the Carolinas in 6 days. My assumptions are usually correct so Its a good bet 60% ormore that this might only bring some squalls at most to the east coast of Florida.

Reasons... Formation of the center is further NE.. Model trend on the ECMWF is more east near Nassau and I think it will end up just east of there heading NNW by Thurs near 25N and 76W and get no closer than 79W making landfall in S Carolina.

I could be wrong... but this was suppose to form around 15N and 63W by Sunday morning.. it's already around 16.2N as of this post and 61W... only chance this has of making Florida is a west turn on Sunday (cause its more WNW right now) and its LLC get pulled W of the midlevel Circulation over Haiti due to landfall around 18N and 70W exiting around Port-A-Prince Haiti and then going inland over SE Cuba keeping this a 50mph Tropical Storm until it comes off around 78W and 23N...but that just probably not going to happen...probably come off near Labadee Haiti moving WNW and moving NW thru middle of the Bahamas.

I will adjust if needed once Irene makes landfall in Hispaniola on Tuesday.

Quoting weatherguy03:TS Irene relocated its center alittle farther North last night. This puts the chances of Irene moving just to the East of Florida alittle higher and ups the chances of a South Carolina/North Carolina landfall paralleling the SE Coast.

Not much intensification has taken place since yesterday. Dropsonde got 1006mb at the surface, but also had 14 knots of wind. Using the rule of thumb (lower the pressure 1mb for every 10 knots), Irene is probably a 1004.6mb cyclone right now.

Which models are "poping" to the west? I see everything has shifted east. Even the GFDL which was a significant outlier has shifted way east into the Eastern GOM. I would expect it to catch up with the other models in the next run or two.

There are 2 models that have the storm heading over cuba into the gulf, just saying I hope models dont trend west, like you said you see them moving east so this wont be a factor in a few more runs.

Quoting Cayman2010:Good Morning. I agree Kman. As always we should watch closely and make no assumptions until it actually makes that turn. Of course, even afterwards we'll be watching closely as many of us have friends over in Florida - just won't have carry out the final stages of prep.

And family there as well. Fortunately for Fla. a track from the Caribbean has some rough terrain to pass over which means that Irene should not be more than a CAT 1 when it gets there depending on when and where it makes the crossing.

A fast forward speed confirms the strength of the ridge and the fact that Irene is right up against the base of it. We could see it run pretty much close to due West for a couple of days. I would not be surprised to see the heading come down to 275 from 280, or a bit flatter.

Quoting TampaSpin:What is scary for the West Coast of Florida is just how long that ridge is holding on........we have seen this too many times when the models shift because they underestimate the ridge. Not saying that is the case here but a storm trucking on at 21mph is a dang strong ridge.....JUST SAYN